Hydrocarbon drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information may relate characteristics of the earth formations traversed by the wellbore, along with the size and configuration of the wellbore itself. The collection of information relating to conditions downhole may be termed “logging.”
Operators often log a wellbore during the drilling process, thereby eliminating the necessity of removing the drilling assembly to insert a wireline logging tool to collect the data. Data collection during drilling also enables the operator to make accurate modifications or corrections as needed to steer the well or optimize drilling performance while minimizing down time. Measurement-While-Drilling (MWD) and/or Logging-While-Drilling (LWD) tools and techniques allow measurement and/or logging of various conditions within the wellbore and/or the surrounding rock formations during drilling operations. MWD/LWD tools may employ a variety of sensors to sample and aggregate digital values for continuous real-time telemetering to the surface during drilling operations. The transmission scheme and channel medium may vary.
Mud pulse telemetry is one of the most common methods used to send data from the bottom of a well to the surface while drilling. Mud pulse telemetry uses drilling fluid that is circulated through the well during drilling as a communication channel. That is, the column of drilling mud that is pumped down through the drill string becomes a medium for sending data. Pressure pulses generated by a telemetry transmitter at a bottom hole assembly (BHA) send information to the well surface. In positive-pulse systems, a valve or other form of flow restrictor creates pressure pulses in the fluid flow by adjusting the size of a constriction in the drill string. In negative-pulse systems, a valve creates pressure pulses by releasing fluid from the interior of the drill string to the annulus. In both system types, the pressure pulses propagate at the speed of sound through the drilling fluid to the surface, where they may be detected various types of transducers.
Data transfer rates in mud pulse telemetry systems tend to be relatively low, on the order of 30 bits per second in shallow wells and ten bits per second in deep wells of actual downhole data. The transmission channel is bandwidth limited and may have noise created by drilling pumps, cutting action of the drill bit, and other sources. The downhole telemetry transmitter may also have physical and electronic constraints, resulting in a low, or even negative (as measured in dB), signal-to-noise ratio (SNR).
Attempts to increase data transfer rates may result in intersymbol interference (ISI), a form of distortion of a signal in which one data symbol interferes with one or more subsequent symbols, causing successive symbols to “blur” together. The presence of ISI in the mud pulse telemetry system may introduce errors at the telemetry receiver.
One way to mitigate the effects of ISI is to apply adaptive equalization, channel tracking, and/or error correcting codes at the telemetry receiver, that, broadly speaking, attempts to undo the effects of the channel by applying an inverse filter. Adaptive equalization and adaptive channel tracking may require a good estimation of the channel impulse response, so that an inverse of the channel impulse response may be applied to the received signal to reconstruct the original signal. A chirp signal, a frequency stepped signal, a pseudo random sequence, or a binary pseudo random signal may be used as a reference to estimate the channel impulse response either in the time domain, using cross-correlation, or in the frequency domain. Divergence of channel tracking or adaptive equalization, with concomitant data errors, may occur if the initial estimate of the channel impulse response is too far from the correct solution.